Catalytic hydroprocessing refers to petroleum refining processes in which a carbonaceous feedstock is brought into contact with hydrogen and a catalyst, at a higher temperature and pressure, for the purpose of removing undesirable impurities and/or converting the feedstock to an improved product.
Hydrocracking is an important refining process used to produce valuable products such as turbine fuel, diesel fuel and lube oil fractions as well as lower boiling hydrocarbonaceous liquids such as naphtha and gasoline, by hydrocracking a hydrocarbon feedstock derived from crude oil.
FIG. 1 is a flow scheme for a typical two-stage, high conversion hydrocracking unit. This particular flow scheme is typically used for hydroprocessing conventional hydrocracker feedstocks, such as heavy gas oils. These feedstocks have high amounts of nitrogen, often between 500 and 2000 ppm and sulfur, often between 0.5 and 3.5 wt %, and a low API, typically between 15 and 20.
In the two-stage hydrocracking scheme illustrated in FIG. 1, a desalted crude oil feedstock 1 is distilled in an atmospheric crude distillation unit 2. The bottoms or residuum 3 from the atmospheric distillation process is then distilled in a vacuum distillation unit 4. Typical vacuum distillation units are operated to deliver a HVGO/residue cut-point of approximately 1,050° F. (566° C.). Higher cut-points (also referred to as deeper cuts) would be beneficial as this would yield a higher volume of HVGO for processing into valuable middle distillate product. However, running the vacuum distillation unit 4 at a higher cut-point means a more disadvantaged feedstock (higher particulates, more sulfur and nitrogen species and heavy polyaromatic hydrocarbons), requiring the downstream hydroprocessing units to run at higher severity levels (higher feed residence time or lower “liquid hour space velocity,” and higher temperatures), lessening the life of the catalysts.
A HVGO cut 5 from the vacuum distillation unit 4 is hydrotreated in a conventional hydrotreating reactor 6, to saturate complex naphthenic and aromatic compounds and reduce feed contaminants such as nitrogen and sulfur which, if left untreated, would otherwise poison downstream hydrocracking catalysts.
The hydrotreated HVGO 7 is then subjected to hydrocracking conditions in a first stage hydrocracker unit 8, followed by atmospheric distillation of the hydrocracked HVGO feedstock 9 in an atmospheric fractionation column 10. In a typical two-stage hydroprocessing unit, the first stage hydrocracker unit 8 is operated at a severity sufficient to achieve a 45-50% conversion.
Light ends 11 and middle distillate products such as naphtha 12, kerosene 13 and diesel 14 are recovered from the atmospheric fractionation column 10, and the atmospheric bottoms fraction 15 is subjected to further hydrocracking conditions in a second stage hydrocracker unit 16. An FCC bleed 17 from the atmospheric bottoms fraction 15 stream is passed to a standard fluidized catalytic cracking (FCC) unit 18. FCC units convert high-boiling, high-molecular weight hydrocarbon fractions of petroleum crude oils into more valuable gasoline 19, olefinic gases used for making alkylate, and other products such as naphtha. Catalysts employed in FCC units are substantially more tolerant of feedstocks containing high amounts of nitrogen, sulfur and polynuclear aromatics (PNAs), as compared to conventional hydrocracking catalysts.
The entire second stage hydrocracker effluent 20 is recycled back to the atmospheric fractionation column 10. This configuration requires the undesirable components (N, S, PNAs) in the atmospheric bottoms fraction 15 to be recycled to extinction within the hydrocracking loop. This limits the types of feedstocks available for use in two-stage hydrocracking units.
Feedstocks most often subjected to hydrocracking are vacuum and atmospheric gas oils and heavy gas oils recovered from crude oil by distillation. A typical gas oil comprises a substantial portion of hydrocarbon components boiling above about 700° F., usually at least about 50 percent by weight boiling above 700° F. A typical vacuum gas oil normally has a boiling point range between about 600° F. and about 1050° F.
Another important refining process is the use of a coker unit to convert the residual oil from a vacuum distillation column or an atmospheric distillation column into C1-C4 light ends, naphtha, light and heavy gas oils, and petroleum coke. The process thermally cracks the long chain hydrocarbon molecules in the residual oil feed into shorter chain molecules leaving behind the excess carbon in the form of petroleum coke.
The residuum of a coker unit, heavy coker gas oil (HCGO), is a high boiling range, highly disadvantaged stream as it contains high amounts of polycyclic aromatics, and nitrogen and sulfur species. Refineries often struggle with finding ways to dispose of their HCGO streams. Some refineries pass HCGO stream on to a fluid catalytic cracking unit (FCC) unit where is upgraded to gasoline and olefinic products. However, if a refinery lacks an FCC unit, other ways of disposing of the HCGO stream must be employed. Accordingly, there is a continuing need for refining processes which allow refiners to process HCGO and other highly disadvantaged streams into more valuable products.
HCGO streams cannot be hydroprocessed using conventional two-stage hydrocracking units. This is because such units are closed looped units which require all of the disadvantaged components (e.g. N, S, PNAs) of the feed to be recycled within the loop to extinction. Because HCGO streams contain very high concentration of disadvantaged components, if a refiner were to introduce a HCGO stream into conventional two-stage hydrocracking units, the catalyst life would be hastened.
Further, conventional two-stage hydrocracking units typically produce middle distillate products listed in the table below.
Conventional Two-Stage Typical Cut Points, ° F. (° C.)Hydrocracking UnitFor North AmericanProductsMarketLight NaphthaC5-180 (C5-82)Heavy Naphtha180-300 (82-149)Jet300-380 (149-−193)Kerosene380-530 (193-277)Diesel530-700 (277-371)
However, middle distillate products are not as profitable for refiners as lube oil products. Therefore, refineries are continuously looking for ways to retrofit existing hydroprocessing equipment to switch from manufacturing a middle distillate slate to a lubricating oil product slate.
Accordingly, there is a continuing need for refining processes which allow refiners to process HCGO and other highly disadvantaged streams, and allow for the manufacture of a higher quantities of highly profitable lubricating oil products.